Beamformer phase optimization for a multi-layer MIMO system augmented by radio distribution network

ABSTRACT

A system for selecting optimal phase combinations for RF beamformers in a MIMO hybrid receiving systems augmented by RF Distribution Network. The system addresses the issue of providing beamforming gains for a plurality of layers using one common set of weights for each beamformer. The specification may be based on channel estimation of all layers as viewed by all receiving antennas, and maximizing metrics that capture the total received power.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. non-provisional patent application Ser. No. 13/776,204 filed on Feb. 25, 2013, which is a continuation-in-part application of U.S. non-provisional patent application Ser. No. 13/630,146 filed on Sep. 28, 2012, which in turn claims benefit from U.S. provisional patent application Nos. 61/652,743 filed on May 29, 2012; 61/657,999 filed on Jun. 11, 2012; and 61/665,592 filed on Jun. 28, 2012; and U.S. non-provisional patent application Ser. No. 13/776,204 further claims benefit from U.S. provisional patent application Nos. 61/658,015 filed on Jun. 11, 2012; 61/658,010 filed on Jun. 11, 2012; 61/658,012 filed on Jun. 11, 2012; and 61/671,416 filed on Jul. 13, 2012, all of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to the field of radio frequency (RF) multiple-input-multiple-output (MIMO) systems and more particularly to systems and methods for RF MIMO systems using RF beamforming and/or digital signal processing, to augment the receiver performance.

BACKGROUND

Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter.

The term “MIMO” as used herein, is defined as the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the transmit power over the antennas to achieve spatial multiplexing that improves the spectral efficiency (more bits per second per Hz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading), or increased antenna directivity.

The term “beamforming” sometimes referred to as “spatial filtering” as used herein, is a signal processing technique used in antenna arrays for directional signal transmission or reception. This is achieved by combining elements in the array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity.

The term “beamformer” as used herein refers to RF circuitry that implements beamforming and usually includes a combiner and may further include switches, controllable phase shifters, and in some cases amplifiers and/or attenuators.

The term “Receiving Radio Distribution Network” or “Rx RDN” or simply “RDN” as used herein is defined as a group of beamformers as set forth above.

The term “hybrid MIMO RDN” as used herein is defined as a MIMO system that employs two or more antennas per channel (N is the number of channels and M is the total number of antennas and M>N). This architecture employs a beamformer for each channel so that two or more antennas are combined for each radio circuit that is connected to each one of the channels.

In hybrid MIMO RDN receiving systems, when the phases of the received signals from each antenna are properly adjusted or tuned with respect to one another, the individual signals may be combined and result in an improved performance of the receiving system.

FIG. 1 shows an example of a standard 2×2 MIMO radio 20 with two antennas A and B communicating with a base station 10 having two transmit antennas radiating Tx1 and Tx2. While each antenna receives both transmitted layers, the baseband separates them and processes them in an optimal way

SUMMARY

Embodiments of the present invention address the challenge of aligning the phases in the receive antennas coupled to the beamformers in the hybrid MIMO RDN architecture, in order to mitigate the combiners losses caused by misaligned phases.

Embodiments of the present invention are based on seeking maximization of the total power received from all transmitted layers as measured by the MIMO's baseband; the summation includes all transmitting antennas signals, as viewed by all receiving RDN antennas, which are equipped with phase shifters.

The received powers may be measured via channel estimation of individual antennas thru their respective beamformers, radios and baseband circuitry.

Different metrics are provided to quantify the said total received power:

${P_{TOTAL} = {\sum\limits_{j = 1}^{M}{\sum\limits_{K = 1}^{L}P_{j,k}}}},$ wherein P_(j,k) denotes power associated with each one of received signals S_(j,k) so that P_(j,k)=[abs(S_(j,k))]² j=1, 2 . . . M, k=1, 2 . . . L.

It would be therefore advantageous to find a way to use a single degree of freedom i.e. the need to choose or select one phase in aligning a beamformer that serves 2, 4, or more different phase setting, stemming from the fact that multiple incoming signals have each a specific possible phase alignment for the beamformer.

The requirement for optimal alignment of phases for all transmitted layers appears also in higher MIMO ranks and in various RDN configurations. A general optimization process is addressed in embodiments of the invention described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and in order to show how it may be implemented, references are made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections. In the accompanying drawings:

FIG. 1 is a high level block diagram illustrating a system according to some embodiments of the prior art;

FIG. 2 is a high level block diagram illustrating a system according to some embodiments of the present invention;

FIGS. 3 and 4 are signal diagrams illustrating an aspect according to embodiments of the present invention;

FIG. 5 is a table with signal diagrams illustrating an aspect according to embodiments of the present invention; and

FIG. 6 is a signal diagram illustrating yet another aspect according to embodiments of the present invention.

The drawings together with the following detailed description make the embodiments of the invention apparent to those skilled in the art.

DETAILED DESCRIPTION

With specific reference now to the drawings in detail, it is stressed that the particulars shown are for the purpose of example and solely for discussing the preferred embodiments of the present invention, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention. The description taken with the drawings makes apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

Before explaining the embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following descriptions or illustrated in the drawings. The invention is applicable to other embodiments and may be practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

FIG. 2 shows an example of a 2×2 MIMO RDN architecture in which each receive antenna as shown in FIG. 1 such as A1, and B1 are enhanced by adding another antenna, A2 and B2 respectively, thus providing reception by four antennas instead of two. The hybrid MIMO RDN architecture further includes phase shifters 40-1 and 40-2 and combiners 30-1 and 30-2.

Without losing generality, and for the sake of simplified explanation, it is assumed herein that the base station transmits each layer over one Tx antenna.

The Hybrid MIMO RDN can provide an additional gain however, as the combiners 30-1 and 30-2 are serving two different Tx antennas with only one phase shifter, it is possible that the diversity parameters (e.g., phase) that are used to optimize the reception of Tx1 are not the same as those needed for receiving Tx2. This is especially true if the antennas are not correlated from one to another.

As seen in FIG. 2, if the phase shift introduced in the path from antenna radiating Tx1 (1LA1-1LA2) is compensated by the phase shifter, that phase shifter setting will only be correct it the paths from the antenna radiating Tx2 are the same. That is, the phase setting will only be correct if (2LA1-2LA2) is the same or a multiple wavelength from (1LA1-1LA2). A similar outcome holds for the Tx1 and Tx2 signals received by Antennas B1 and B2.

If the case using four 90° phase shifts is compared to align the signals from Tx1, it is apparent that there are three possible outcomes for the Tx2 signal:

The first outcome is that the signals arrive at the antennas A1 and A2 with a similar phase differences as for the Tx1 transmission so the same phase setting used to enhance the reception of Tx1 will also enhance Tx2. (25%);

The second outcome is that the resulting Tx2 signals to A1 and A2 are +/−90° from each other and will produce zero diversity gain for this process. (50%); and

The third outcome is that the resulting Tx2 signals are 180° from each other and can cancel each other or produce a negative diversity gain depending on their relative amplitudes. (25%).

When the result is the aforementioned third outcome, the system must choose to sacrifice diversity gain for Tx1 in order to avoid the total loss of the Tx2 signal. This would result in low diversity gain (˜0 dB) for both Tx1 and Tx2.

The algorithm offered by embodiments of the invention results in phase optimization based on seeking maximization of the total power received from all transmitted layers as measured by the MIMO's baseband; the summation includes all transmitting antennas signals, as viewed by all receiving RDN antennas, which are equipped with phase shifters. The aforementioned received powers are measured via channel estimation of individual antennas thru their respective beamformers, radios and baseband circuitry.

In accordance with some embodiments of the present invention, a multiple inputs multiple outputs (MIMO) receiving system having number N channels is provided. The MIMO receiving system may include a radio distributed network (RDN) having number N beamformers, each having number K_(N) antennas. The MIMO system may further include at least one phase shifter associated with one or more of the N beamformers. Additionally, the MIMO receiving system is configured to: (a) select one phase that optimizes performance of multiple layers, via channel estimation of each layer as seen (e.g, taking into account the gain and phase affected by the physical location) by each receiving antenna, and (b) maximize a total received power from all transmitted signals.

FIGS. 3 and 4 are signal diagrams illustrating an aspect according to embodiments of the present invention. In the following non-limiting example, a case of N plurality of uncorrelated transmit signals projected from a base station, where N=2, is received by a 2×2 MIMO UE which is augmented by an RDN with 2 beamformers, each beamformer has three receive antennas. It is assumed, for the sake of the following example that the beamformers can select one of 4 possible phases: 0°, 90°, 180°, and 270°. When selecting the beamformers' phases is such a way that will maximize the received signal coming from Tx1, the Tx 2 phases may or may not be constructively combined, but rather, may have 1*4*4=16 phase combinations.

For the sake of simplicity, it is assumed that each receive antenna provides the same amplitude and a randomly selected phase out of 4 alternatives. It is also assumed that the amplitudes power is 0.33 (for the sake of the example).

As the signals are fed into an RF combiner, the translation into voltage of each signal provides a combined result as described herein below:

In FIG. 3-A, 3 aligned vectors, each of ⅓ of a Watt are depicted so that the combined voltage equals 3×SQRT (0.333)=1.732 V=>3 W. Since the gain is the output divided by the input, the gain here equals 3 W/1 W=3, hence 4.77 dB. FIG. 3-B depicts two aligned vectors each of ⅓ of a Watt and a single ⅓ Watt perpendicular vector so that the combined voltage equals SQRT (0.33)+j SQRT (0.33)=>Effective voltage combining=SQRT [(2×0.577)²+0.577²]=>1.67 W. For a similar reason, the gain equals 1.67 W/1 W=1.67 hence 2.2 dB. Similar calculation for FIGS. 3-C, D, E, F, G generates the gains of 2.2 for each one. Applying similar calculation for FIGS. 3-H, I, J, K, L, M, N, O, P generates the gains of −4.77 dB for each one.

As can be seen, the seven combinations described in FIG. 3-A through G provide positive gains for both layers, while the nine combinations described by FIG. 4-H through P, provide positive gain for one layer and negative gain for the other.

FIG. 5 demonstrates in form of a table and a corresponding signal diagram how theses conclusion have been reached. Specifically, table 500 illustrates the aforementioned calculations with specific configurations 501-505.

It can be easily seen that while configuration 501 yields 4.77 dB gain, configuration 502 yields a lesser yet still positive gain of 2.22 dB. Configurations 503-505 on the other hand, yield a negative gain of −4.77 dB.

As can be seen above in FIGS. 3 and 4 when aligning one transmit signal to best gain, the second transmit signal is left for random combination of phases, and may become exceedingly adverse at many of the cases.

FIG. 6 illustrates improvements that can be achieved by embodiments of the present invention in overall gain terms. The upper part illustrates cases where the selection of a maximal gain for Tx 1, where all antennas are aligned, undermines the gain for Tx 2. The lower part of FIG. 6 demonstrates that replacing +4.77 dB gain for layer 1 and −4.77 dB for layer 2 provides gains of +2.22 dB for both layers at 9 out of 16 of the cases; (it is noted that in other 6 cases, the corresponding gains are +4.77 dB and 2.22 dB, and in one other case both are +4.77 dB)

Similar approaches can be applied to more complex MIMO hybrid RDN configurations, where there are more layers and or more antennas are combined by RF beamformers.

One embodiment of metrics and a procedure for the selection of optimal phase settings to all participating beamformers is described below:

Consider a beamformer with K_(N) receive antennas, each of them receiving signals from N transmit antennas. The channel functions h_(i,j,k) from transmit antenna j, j=1, 2 . . . N, to receive antenna i, i=1, 2 . . . K_(N), at frequency k, k=1, 2 . . . L (it is assumed the general case of frequency selective channels) are obtained through channel estimation done by the base-band.

Each receive antenna is equipped with a set A of R phase shifters, }, for phase adjustment. The set A of phase shifters could be, for example, to, {0, 90, 180, 270} degrees. The algorithm needs to select the optimal phase φ_(i)εA to be applied to receive antenna: i, i=1, 2 . . . K_(N).

After phase adjusting the K_(N), receive antennas, the overall channel functions seen by the receiver under consideration are:

${S_{j,k} = {\sum\limits_{i = 1}^{K_{N},}{h_{i,j,k}{\mathbb{e}}^{j\;\Phi_{i}}}}},{j = 1},{2\mspace{14mu}\ldots\mspace{14mu} N},{k = 1},{2\mspace{14mu}\ldots\mspace{14mu}{L.}}$

A power P_(j,k) is associated with each one of them:

P_(j, k) = [abs (S_(j, k))]², j = 1, 2  …  N, k = 1, 2  …  L.

In one embodiment the algorithm selects phases φ_(i)εA, i=1, 2 . . . K_(N′), so as to maximize the total power P_(Total) defined as:

$P_{TOTAL} = {\sum\limits_{j = 1}^{N}{\sum\limits_{K = 1}^{L}{P_{j,k}.}}}$

In another embodiment, a procedure and metrics is provided wherein the antennas, e.g., antenna phases, are adjusted one by one recursively. As before, φ₁ may be set to zero. To calculate φ₂ the contributions from only h_(1,j,k) and h_(2,j,k) are considered. The combined channel s_(2,j,k) and channel power p_(2,j,k) for the first two antennas are defined as: S _(2,j,k) =h _(i,j,k) e ^(jφ) ¹ +h _(2,j,k) e ^(jφ2) j=1,2 . . . N, k=1,2 . . . L p _(2,j,k)=[abs(S _(2,j,k))]² ,j=1,2 . . . N, k=1,2 . . . L The algorithm selects or chooses φ₂εS that maximizes

$\sum\limits_{j = 1}^{N}{\sum\limits_{k = 1}^{L}\;{p_{2,j,k}.}}$ Continuing in a similar fashion for all antennas, once φ_(i-1) has been calculated or determined, φ_(i) is calculated. Define:

${s_{i,j,k} = {\sum\limits_{l = 1}^{i}{h_{l,j,k}{\mathbb{e}}^{{j\Phi}_{l}}}}},{j = 1},{2\mspace{14mu}\ldots\mspace{14mu} N},{k = 1},{2\mspace{14mu}\ldots\mspace{14mu} L},{p_{i,j,k} = \left\lbrack {{abs}\left( s_{i,j,k} \right)} \right\rbrack^{2}},{j = 1},{2\mspace{14mu}\ldots\mspace{14mu} N},{k = 1},{2\mspace{14mu}\ldots\mspace{14mu}{L.}}$

Then, similarly, the algorithm selects or chooses φ_(i)εS that maximizes

$\sum\limits_{j = 1}^{N}{\sum\limits_{k = 1}^{L}\;{p_{i,j,k}.}}$

The total number of possible antenna phase combinations for the recursive algorithm is R (K_(N)−1).

Since the order in which the antennas are optimized may affect the outcome, some criterion may be used for numbering of the antennas. For example, in some embodiments the antennas may be sorted or ordered in ascending/descending order based on the total power P_(Ant) _(i) , of each antenna:

${P_{{Ant}_{i}} = {\sum\limits_{j = 1}^{N}{\sum\limits_{k = 1}^{L}\left\lbrack {{abs}\left( h_{i,j,k} \right)} \right\rbrack^{2}}}},{i = 1},{2\mspace{14mu}\ldots\mspace{14mu}{K_{N}.}}$

By repeating the aforementioned process for all beamformers in the hybrid MIMO RDN system an optimized overall gain for the entire hybrid MIMO RDN architecture is achieved.

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or an apparatus. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.”

In various embodiments, computational modules may be implemented by e.g., processors (e.g., a general purpose computer processor or central processing unit executing code or software), or digital signal processors (DSPs), or other circuitry. The baseband modem may be implanted, for example, as a DSP. A beamforming matrix can be calculated and implemented for example by software running on general purpose processor. Beamformers, gain controllers, switches, combiners, and phase shifters may be implemented, for example using RF circuitries.

The aforementioned flowchart and block diagrams illustrate the architecture, functionality, and operation of possible implementations of systems and methods according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions.

In the above description, an embodiment is an example or implementation of the inventions. The various appearances of “one embodiment,” “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments.

Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment.

Reference in the specification to “some embodiments”, “an embodiment”, “one embodiment” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the inventions.

It is to be understood that the phraseology and terminology employed herein is not to be construed as limiting and are for descriptive purpose only.

The principles and uses of the teachings of the present invention may be better understood with reference to the accompanying description, figures and examples.

It is to be understood that the details set forth herein do not construe a limitation to an application of the invention.

Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above.

It is to be understood that the terms “including”, “comprising”, “consisting” and grammatical variants thereof do not preclude the addition of one or more components, features, steps, or integers or groups thereof and that the terms are to be construed as specifying components, features, steps or integers.

If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element.

It is to be understood that where the claims or specification refer to “a” or “an” element, such reference is not be construed that there is only one of that element.

It is to be understood that where the specification states that a component, feature, structure, or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included.

Where applicable, although state diagrams, flow diagrams or both may be used to describe embodiments, the invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described.

The descriptions, examples, methods and materials presented in the claims and the specification are not to be construed as limiting but rather as illustrative only.

Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined.

The present invention may be implemented in the testing or practice with methods and materials equivalent or similar to those described herein.

While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents. 

We claim:
 1. A multiple inputs multiple outputs (MIMO) receiving system receiving N channel layers transmitted from N transmit antennas, comprising: a radio distributed network (RDN) having N beamformers, N being a positive integer greater than 1, each of the N beamformers having K_(N) antennas comprising one primary receive antenna and K_(N)−1 additional receive antennas, K_(N) is a positive integer greater than 1; K_(N)−1 phase shifters within each of the N beamformers configured to phase shift the received signals from each of the K_(N)−1 additional receive antennas within each of the N beamformers; N combiners respectively associated with the N beamformers, each of the N combiners is configured to combine the K_(N)−1 phase shifted signals and the received signal from the primary antenna for each of the N beamformers; radio and baseband circuitry configured to receive the combined signals from each of the N combiners; and the MIMO receiving system is configured to select K_(N)−1 phases for the K_(N)−1 phase sifters in each of the N beamformers that: (a) optimizes performance of the N channel layers via channel estimation of each of the N layers as seen by the radio and baseband circuitry, and (b) maximizes a total received power from all transmitted signals received at the MIMO receiving system by all of the receive antennas.
 2. The system according to claim 1, further comprising: for each of the N beamformers: (i) calculating channel functions h_(i,j,k) from each one of the N transmit antennas j, j=1, 2 . . . N, to each one of the K_(N) receive antennas i, i=1, 2 . . . K_(N) at frequency k, k=1, 2 . . . L, at a baseband module using channel estimation, and (ii) selecting phases, wherein A={φ₁, φ₂ . . . φ_(R)} so as to maximize a total power P_(Total) defined as: ${P_{Total} = {\sum\limits_{j = 1}^{N}{\sum\limits_{k = 1}^{L}P_{j,k}}}},$  wherein P_(j,k) denotes power associated with each one of received signals s_(j,k) wherein ${s_{j,k} = {\sum\limits_{i = 1}^{K_{N}}{h_{i,j,k}{\mathbb{e}}^{{j\Phi}_{i}}}}},$  j=1, 2 . . . N, k=1, 2 . . . L so that P_(j,k)=[abs(S_(j,k))]², j=1, 2 . . . N, k=1, 2 . . . L; and repeating the calculating and the selecting stages for each of the N beamformers.
 3. The system according to claim 1, further comprising: for each of the N beamformers: adjusting each of the K_(N) antennas one by one, recursively, wherein φ₁ is set to zero, and only contributions from h_(1,j,k) and h_(2,j,k) are used to calculate φ₂, defining a combined channel S_(2,j,k) and a channel power p_(2,j,k) for two antennas adjusted first as: p_(2,j,k)=[abs(s_(2,j,k))²], j=1, 2 . . . N, k=1, 2 . . . L, and choosing φ₂εA that maximizes $\sum\limits_{j = 1}^{N}{\sum\limits_{k = 1}^{L}\;{p_{2,j,k}.}}$
 4. The system according to claim 3, wherein the optimization of the phase of the beamformer is calculated, wherein, once φ_(i-1) has been determined, φ_(i) is calculated.
 5. The system according to claim 4, wherein: ${s_{i,j,k} = {\sum\limits_{l = 1}^{i}{h_{l,j,k}{\mathbb{e}}^{{j\Phi}_{l}}}}},{j = 1},{2\mspace{14mu}\ldots\mspace{14mu} N},{k = 1},{2\mspace{14mu}\ldots\mspace{14mu} L}$ p_(i, j, k) = [abs(s_(i, j, k))]², j = 1, 2  …  N, k = 1, 2  …  L and wherein the φ_(i)εA that maximizes $\sum\limits_{j = 1}^{N}{\sum\limits_{k = 1}^{L}\;{p_{i,j,k}.}}$
 6. The system according to claim 4, wherein: Φ_(i) εA A={φ ₁,φ₂ . . . φ_(R)} $P_{Total} = {\sum\limits_{j = 1}^{N}{\sum\limits_{k = 1}^{L}P_{j,k}}}$ $S_{j,k} = {\sum\limits_{i = 1}^{K_{N}}{h_{i,j,k}{\mathbb{e}}^{{j\Phi}_{i}}}}$ j = 1, 2  …  N, i = 1, 2  …  K_(N), k = 1, 2  …  LP_(j, k) = [abs(S_(j, k))]², j = 1, 2  …  N, k = 1, 2  …  L. 